1. Field of the Invention
The present invention relates to a remote supervisory control system including a master station and a plurality of slave stations sharing transmission paths.
2. Description of the Prior Art
Conventionally, in a remote supervisory control system including a master station and slave stations sharing transmission paths, means adopted in most cases as a method to avoid an interference due to simultaneous call initiations from a plurality of slave stations is a polling system in which the master station sequentially calls the slave stations. An example of codes to be exchanged between the master station and slave stations in such a case is described in pages 33 to 35 of the "IEEE TUTORIAL COURSE Fundamentals of Supervisory Control Systems".
FIG. 1 is a block diagram, for example, showing an example of the constitution of the conventional remote supervisory control system which exchanges codes as described in the literature above. In this diagram, 0 is a master station, 1, 2, . . . , and N indicate slave stations, 01, 10, 12, 21, 23, 32, . . . , MN, and MN are transmission paths or routes connecting between the master station 0 and the respective slave stations 1, 2, . . . , and N.
In the master station, 0, 001 indicates a central processing circuit, 002 denotes a display output circuit, 003 represents an input circuit to input signals to the central processing circuit 001, 004 indicates a display panel to display the contents of outputs from the display output circuit 002, 005 denotes a control console, 006 designates a code transmission circuit, and 007 represents a modulation circuit to modulate an output from the code transmission circuit 006.
The constitution further includes a code reception circuit 008 and a demodulation circuit 009 which demodulates a signal transmitted from the slave station 1 through the transmission route 10 and sends the resultant signal to the code reception circuit 008.
Furthermore, in the slave station 1, 100 stands for a central processing circuit, 101 indicates an output circuit to output signals to pertinent devices, 102 designates an input circuit to input signals from the pertinent devices, 103 denotes a branch circuit to effect a branching of a modulated signal delivered via the transmission path 01 between a descending signal amplifier circuit 104 and a demodulation circuit 105.
Reference numeral 106 stands for a code reception circuit and 107 denotes a code transmission circuit which encodes, based on a instruction from the central processing circuit 100, signals inputted to the input circuit 102. Reference numeral 108 indicates a modulation circuit which modulates an output from the code transmission circuit 107 in accordance with an instruction from the central processing circuit 100. Reference numeral 109 denotes an insertion circuit and 110 stands for an ascending signal amplifier circuit. The insertion circuit 109 inserts an output from the modulation circuit 108 and outputs the resultant signal to the ascending signal amplifier circuit 110.
The configuration of the components in each slave station ranging from the sleeve station 2 to the slave station N is completely the same as the slave station 1, and the hundreds digit of the number assigned to each element represents a slave station number, whereas the tens digit and the units digit designate the corresponding circuit.
Although a descending signal amplifier circuit N04 associated with the slave station N is unnecessary at an end position, it is convenient to provide the descending signal amplifier circuit N04 for the standardization of the apparatus and for an addition of a slave station in the future.
Next, the operation of FIG. 1 will be described by also referring to the timing chart of FIG. 2. In the master station 0 of FIG. 1, the central processing circuit 001 instructs the code transmission circuit 006 to sequentially transmit a polling signal to the respective slave stations 1 to N in a continual fashion, and the code transmission circuit 006 passes a polling signal by serial codes to the modulation circuit 007 based on the instruction.
The modulation circuit 007 receives the signal and effects a modulation such as a frequency shift keying, FSK modulation so as to transmit the resultant signal via the transmission path 001 to the slave station 1.
In the slave station 1, the branch circuit 103 receives the signal and achieves a branching operation thereof to the demodulation circuit 105 and the descending signal amplifier circuit 104.
The demodulation circuit 105 demodulates a received signal and supplies the resultant signal to the code reception circuit 106. On the other side, the descending signal amplifier 104 amplifies the signal undergone attenuations in the transmission path 01 and the branch circuit 103 to a predetermined level and sends the obtained signal via the transmission path 12 to the slave station 2.
Also in the slave station 2, the same operation is accomplished as that effected in the slave station 1; furthermore, in the similar fashion, the polling signal transmitted from the code transmission circuit 006 of the master station 0 is received by the code reception circuits 106, 206, . . . , N06 of the respective slave stations 1 to N.
The polling signal sent from the code transmission circuit 006 of the master station 0 is as shown in FIG. 2 (00) which includes a synchronization bit as the first information followed by an address (A.sub.1, A.sub.2, etc.) specified, a function code (F indicating a polling signal in this case), an error detection code S, and a code end bit as the final information.
When the code transmission circuit 006 of the master station 0 of FIG. 1 sends a polling signal shown in the upper-left corner of FIG. 2 (00), the polling signal reaches the code reception circuit 106, 206, . . . , N06 of the respective slave stations 1 to N as described above; however, since an address part A.sub.1 specifies the slave station 1, the central processing circuits 200, . . . , N00 of the other slave stations 2 to N decode the address and do not carry out the transmission, namely, only the central processing circuit 100 of the slave station 1 first sends a carrier wave of the modulation circuit 108 and subsequently causes the code transmission circuit 107 to transmit a response signal to the master station 0.
At the present stage, since any particular status change is not found, the response signal only includes an address A.sub.1 of the own location and a function code (F indicating that there does not exist any status change in this case) as shown in FIG. 2 (01). This code information reaches the demodulation circuit 009 of the master station 0 through the insertion circuit 109, the ascending signal amplifier circuit 110, and the transmission path 10 and is then demodulated therein so as to be supplied to the code reception circuit 008.
On receiving the demodulated signal, the code reception circuit 008 sends an interruption signal to the central processing circuit 001, which in response to the interruption signal reads the received content of the code reception circuit 008.
Since the received content thus read out does not particularly indicate a fact that a status change has taken place, the central processing circuit 001 instructs the code transmission circuit 006 to send a polling signal specifying the slave station 2.
The code transmission circuit 006 converts the signal into serial codes [refer to FIG. 2 (00)] and sends the codes via the modulation circuit 007 to the transmission route 01. This polling signal also reaches the code reception circuit 106, 206, . . . , N06 of the respective slave stations 1 to N, like in the case of the specification of the slave station 1 described above; however, since the address part A.sub.2 specifies the slave station 2 in this case, the central processing circuit 200 of the slave station 2 first causes the carrier wave of the modulation circuit 208 to be transmitted and then causes the code transmission circuit 207 to send a response signal to the master station 0.
Since there is not found any particular status change in the slave station 2 in this situation, the signal includes, like in the case of the slave station 1, only the address A.sub.2 of the own location, the function code F, and the error detection code S. [Refer to FIG. 2(02) for details.]
When the response signal is received via the code reception circuit 008, the central processing circuit 001 of the master station 0 then instructs the code transmission circuit 006 to send a polling signal specifying the slave station 3.
The polling operation is repeatedly effected in the similar fashion up to the slave station N as shown in FIG. 2(00) and FIG. 2(01) to (0N), and then the polling operation returns to the slave station 1 again. If there does not exist any status change in the slave stations, the operations above are repeatedly achieved in a cyclic fashion.
Under the condition above, in a case where a status change (such as an automatic trip of a breaker) occurs in the slave station 1, as shown in the codes from the upper-right corner to the lower-left corner of FIG. 2, when a polling signal from the master station 0 specifies the slave station 1 (by use of the address A.sub.1), the central processing circuit 100 of the slave station 1 instructs the code transmission circuit 107 to set the function code F to notify an occurrence of the status change and to subsequently transmit the data (the position number and the direction of the change of the device associated with the status change).
On receiving the signal of the instruction above, the code transmission circuit 107 sends the codes shown in FIG. 2(01) to (11). When the code reception circuit 008 receives this information in the master station 0, the central processing circuit 001 reads the information and then conducts a warning operation via the display output circuit 002, for example, to light and flicker the pertinent display lamp of the display panel 004 and to sound a bell.
After this operation, the central processing circuit 001 instructs the code transmission circuit 006 to send a polling signal specifying the slave station 2 and then continuously achieves the polling operation for the other slave stations.
Next, in a case where the operator at the control console 005 effects an operation to control a device of the slave station N, the central processing circuit 001 reads the operation input via the input circuit 003 and instructs the code transmission circuit 006 to send the control instruction data (the position number and the direction of the control of the pertinent device) with the slave station address code set as N and the function code F set as the control instruction.
On receiving the instruction, the code transmission circuit 006 sends the code shown in FIG. 2(10), namely, shown in the central position of the top of lower lines. Like in the case of the polling signal, these codes are also received by the code reception circuits 106, 206, . . . , N06 of the respective slave stations; however, the central processing circuit N00 of the slave station N detects that the instruction is transmitted to the own station and then outputs the control instruction via the output circuit N01 to the pertinent device based on the data portion of the received codes.
As a result, when the pertinent device issues a response, the result of the response is inputted to the input circuit N02, and then the central processing circuit N00 reads the result and instructs the code transmission circuit N07 to send codes notifying the status change associated with the response to the master station 0. On receiving the instruction, the code transmission circuit N07 sends the codes shown in FIG. 2(1N).
In the master station 0, the code reception circuit 008 receives the codes and then lights, like in the case of the state change described above, the pertinent display lamp of the display panel 004 via the output circuit 003 so as to display the response of the pertinent device to the operator.
In this case, since the central processing circuit 001 has stored information that the control instruction has been sent to the pertinent device, the status change thus received is regarded as a response thereto and consequently the central processing circuit 001 does not carry out the warning operation such as for the flicker of the display light and the sounding of a bell.
Thereafter, the central processing circuit 001 instructs the code transmission circuit 006 to send a polling signal for the slave station 1 and then returns to the ordinary state to effect the polling to the respective slave stations.
Incidentally, in a case where a considerable period of time is required for the response of the pertinent device in the slave station N, the central processing circuit N00 of the slave station N first may issue a response that the control instruction has been received and the response of the pertinent device is carried out when the next polling signal is received.
Furthermore, so-called "two action control method is commonly used in which the control operation is not achieved at a time, namely, a selecting operation of a device is first conducted and then after the selecting operation is confirmed, the control instruction is transmitted.
Since the conventional remote supervisory control system is constituted as described above, the master station 0 sequentially sends the polling signal to the respective slave stations 1 to N, which then effect operations responsive to the polling signal; consequently, a long period of time is required to elapse since a slave station has a chance to communicate with the master station until the slave station has the next chance to achieve a communication therewith. As a result, when a status change takes place immediately after the communication chance, the pertinent slave station must wait for the next chance for a quite a long period of time, which leads to a problem that the display and the warning report associated with the status change are delayed at the master station.
On the other hand, there has been adopted, as a method not having a disadvantage inherent to the polling system above, a so-called token passing method. (For details, refer to the Automatisierungstechnische Praxis apt, 28, Jahrgang, Heft 9, 1986 Sec. 2,c) Token Ring and d) Token Bus.) However, as also described in the literature above, this method is suitable for a case where the transmission paths are formed in a ring shape or a bus shape, and the functions of all stations are equalized excepting the function to monitor whether or not the transmission is being correctly carried out; consequently, although this method is suitable for a local area network (LAN) system in which the transmission routes are disposed in a ring form or a bus form shared in the private system location, there remains a problem that this system is not suitably applicable to a remote supervisory control system in which transmission routes are connected in a linear form or in a branching configuration through a long distance in an ordinary case and the master station and the slave stations are functionally separated from each other to achieve contrastive operations.